IHP-V Projects 2.312.4 Ecohydrology

(1)

INTERNATIONAL HYDROLOGICAL PROGRAMME

Ecohydrology

Science and the sustainable management

of tropical waters

A summary of the projects presented to the Conference Naivasha, Kenya, 1 l-l 6 April 1999

Edited by David Harper Maciej Zalewski

Technical Editor

Malgorzata Lapinska

IHP-V Projects 2.312.4

IHP-V 1 Technical Documents in Hydrology 1 No. 46

Prepared and published in co-operation with the UNESCO Venice Office UNESCO, Paris, 2001

(2)

The designations employed and the presentation of material throughout the publication do not imply the expression of any opinion whatsoever on the part of UNESCO concerning the legal

status of any country, territory, city or of its authorities, or concerning the delimitation of its frontiers or boundaries.

(3)

UNESCO IHP ~ V 2.3/2.4 ECOHYDROLOGY - Science and the Sustainable Management of Tropical Waters

CONTENT

Preface.. ... .5

SECTION 1: Lake Naivasha and its Basin, Eastern Rift Valley, Kenya.. ... .9

SECTION 2: Ecohydrology in Africa.. ... .59

SECTION 3: Ecohydrology - Related Projects ... 93

(4)

WESCO IHP I’2 30.4 ECOHYDROLOGY - Scrence and the Sustarnable Management of Troprcal Waters

PREFACE

The management of waters in the tropics is beset by difficulties more severe than those in the temperate zone. The moist tropics - wet rainforest and high altitude areas - experience a surplus of water which is distant from human settlement uses, and as a consequence water systems are relatively natural but their problems relate to reservoir and river regulation schemes. The arid tropics in contrast, are often those areas where human development is concentrated, increasing pressure upon limited natural water cycles and water stores through pollution and regulation as well as creating new ones.

In temperate zone countries, industrial and agricultural development over the past hundred years has created similar problems which resulted in inadequate water quantity or water quality, and the societies which recognised them have made some strides towards trying to solve them. In the past fifty years there has been an increasing awareness among human populations concentrated in town- and city- dominated states of the value of the natural world and the importance of understanding the processes which underlie its stability. In water management, the most recent way of thinking, offering the greatest prospect of long-term sustainable success, is Ecohydrology. Put simply, this paradigm regards a water catchment more like a Platonian ‘superorganism’, which has properties of resistance and resilience against stress that can be used as sustainable management tools in the future.

The range of papers presented here illustrate the problems inherent in the management of tropical waters but they also illustrate the additional work which is necessary to put Ecohydrological principles in place. It is for that reason that the UNESCO IHP 2.3/2.4 activities supported the conference, promoted the concept of its activities and its Advanced Study Courses (Europe 1999, Africa 200 1) and increased its network of active scientists.

The publications in this Technical Document introduce the range of activities which are being developed in the tropics aimed at Sustainable Management of catchment systems.

They illustrate the progress which is being made towards an holistic style of management of water systems and they demonstrate the extent to which Ecohydrology can help in long-term sustainable management solutions.

David Harper, Macigj Zalewski

D. Harper

University of Leicester Leicester. LE 17RH UK

M. Zalewski International Centre for Ecology, Polish Academy of Sciences, Warsaw

Marii Konopnickie.; I, 05-092 Lomianki, Poland Department of Applied Ecology, University of Lodz Banacha 12/16, 90-237 Lodz, Poland

(5)

UNESCO IHP - 1’2.312.4 ECOHYDROLOGY - Science and the Sustainable Management of Tropical Waters

Science and the Sustainable Management

of Tropical Waters

(6)

UNESCO IHP V2 312.4 ECOHYDROLOGY Science and the Sustarnable Management of Tropical Waters

SECTION 1 LAKE NAIVASHA AND ITS BASIN,

EASTERN RIFT VALLEY, KENYA

(7)

UNESCO IHP V 2.312.4 ECOHYDROLOGY Science and the Sustarnable Managemenr of Troprcal Waters

Preface

Environmental Controls on the Functioning of Shallow Tropical lakes

Shallowness and tropical@ primarily relate to the physical aspects of environmental regulation. These concern water input and output, with correlates of water level and salinity;

energy balance and heat distribution, with correlates of temperature and density; and largely wind-driven water movements, with consequences in chemical and biological distributions.

Shallowness in a water column affects the quantitative relationship between many stock quantities and flux-rates per unit surface area. Evaporative loss of water is one familiar example; sensitivity to surface energy exchanges provides others. Somewhat different are processes that depend on transmission with depth. Here light penetration, convective penetration and wind-generated turbulence/flow depth relations are illustrative.

Tropicality further influences through climatic factors, especially of rainfall and radiation.

Energy balance tends to year-long elevated water temperatures at all depths, but at a level dependent upon altitude. The magnitude and seasonal periodicity of water input is dependent upon the intertropical convergence zone in atmospheric circulation. In the semi-arid and arid tropics the lakes may lie in closed drainage basins and be influenced by evaporative concentration with salinization.

Jack Talling, FRS

Freshwater Biological Association Cumbria, UK

In many of these pages, shallow lakes of mainly tropical Africa illustrate these varied environmental constraints and some biological consequences. Many pages also show, in order to conserve this varied biology for future generations, the directions in which we must travel and the means that we must use, to achieve sustainable management - the integration of a resource’s utilisation with its intact ecological functioning.

This is also the goal of the new paradigm of Ecohydrology - using the properties of an ecosystem to direct its sustainable management. This volume brings Ecohydrology to tropical ecosystems for the first time. We hope that the readers of its pages will find as much to benefit them as the participants in the conference did.

David Harper and Maciej Zalewski

The Editors

(8)

UNESCO IHP ~ V2.3/2.4 ECOHYDROLOGY Science and the Sustainable Management of Tropical Waters

PREFACE

Science and Sustainable Management of Tropical Waters:

The Conference at Naivasha, Kenya, April 11-16th 1999

The conference was originally conceived, in 1997, by Lord Enniskillen (Chairman of the Lake Naivasha Riparian Association) and by David Harper (Principal Investigator of the Earthwatch-Institute funded Naivasha Research Group) as a means of jointly bringing the scientific research results and the achievements of Naivasha’s Ramsar Site Management Plan together to the scientific and political communities and the general public. One of the key pillars of the Plan is to ensure that it’s principles are based on current scientific wisdom. It was an unexpected bonus when in 1998, just as conference plans had entered their final phase, it was announced that the LNRA would be awarded the 1999 Ramsar prize (jointly with the management organisation at Lake Prespa, Greece/Albania). This was subsequently presented at a ceremony in Costa Rica, in May 1999.

The conference was opened by Dr Richard Leakey, Head of Kenya Wildlife Services.

He is a former palaeontologist who first had first been appointed to head KWS a decade ago when poaching threatened to drive elephant and rhino populations in east Africa to extinction and was then re-appointed in 1998. Several politicians including members of the National Assembly Committee on Agriculture and Water Resources attended the opening ceremony and heard, besides Dr Leakey, Professor Maciej Zalewski introduce the concept of ecohydrology and its importance to an understanding of the sustainable exploitation of water resources; and Dr Dirk Vershuren, from Belgium, deliver a timely reminder of the unpredictable climate of tropical regions through his paleo-ecological studies of the last few hundred years at Oloidien lake, Naivasha..

This conference was the first step in taking the Vth International Hydrological Programme “Ecohydrology” of UNESCO to tropical Africa - and 201 delegates from 26 countries participated in four days of scientific presentations and discussion which ranged from reservoirs of north-east Brazil through inland and coastal waters of Africa to shallow floodplain lakes of Indonesia. This diversity of geographical coverage of the conference theme was not the only strength of the meeting. Others were the diversity of approaches to the problems of shallow waters conservation and not least the presentation styles of the speakers.

Few delegates would have imagined in advance that one presenter’s representation of wetland conservation in the song of the community’s youth group could have generated as much interest as the computer-driven presentation of the first successful modelling of Lake Naivasha’s water level fluctuations,

The conference confirmed the international significance of Lake Naivasha and its

catchment in the scientific context, as well as the conservation context recognised by the

Ramsar award, with over 40 presentations of work carried out on the lake and its basin (40 of

them reproduced here). The proceedings were formally closed by Dr Klaus Topfer, head of

the United Nations Environment Programme, based in Nairobi, with Dr Leakey. Dr Topfer

stressed the continued vital importance of water in development but the growing realisation

that the world does not have enough available freshwater: the balance between human needs

and nature’s needs will be a crucial one in the next century. To meet these problems the

concepts of ecohydrology have a central role to play.

(9)

UNESCO IHP - F2.3/2.4 ECOHYDROLOGY - Science and the Sustainable Management of Tropical Waters

Sponsors beside UNESCO IHP V were:

USAID UNDP

Lake Naivasha Riparian Association Kenya Wildlife Service

Kenya Airways

Rockefeller Foundation Earthwatch Institute ITC, Netherlands Monsanto (Kenya) Ltd.

UNEP Kijabe Ltd.

Barclays Bank Kenya Zeneca (Kenya) Ltd.

Kenya Shell Ltd.

African Conservation Centre

NeDA (Netherlands Development Agency) University of Leicester.

David Harper and Andrew Enniskillen

(10)

UNESCO IHP-V 2.3/2.4 ECOHYDROLOGY - Core Study Lake Narvasha

THE ECOHYDROLOGICAL

APPROACH AT LAKE NAIVASHA, KENYA

Author

DAVID HARPER Department of Biology, University of Leicester, UK Study Area

Lake Naivasha is the only freshwater lake in the chain of water bodies which run down the Eastern (Gregory) Rift valley through Kenya - the others are either moderately (Lake Turkana), or highly (Lake Nakuru), alkaline. With an area currently around 100 km*, it is the largest body of freshwater (after the Kenyan share of Lake Victoria). Its freshness is maintained by high altitude rainfall upon the Nyandarua (Aberdare Mountains) rising in excess of 3500 m. on its north-western watershed.

LAKE NAIVASHA AND ITS CATCHMENT

Introduction

The lake was well explored by early scientific expeditions in the 1920s and 193Os, and studies of its limnology started from the University of Nairobi in the 1970s. The high biodiversity of wetland plants in its ecotone, aspects of its water balance and the primary productivity of its basins in relation to their alkalinity were published at that time.

This section of the Technical Document summarises the research that has been carried on since 1982 at Naivasha in co-operation with the University of Nairobi, National Museums of Kenya and the Lake Naivasha Riparian Association. Much of the research has been supported by the Earthwatch Institute, Boston and Oxford, and aided by Earthwatch volunteers, since 1987. We have been able to always collaborate with, and some times aid, the work of the numerous Kenyan organisations which is also described on the following pages, to build up one of the most comprehensive pictures of a lake basin in tropical Africa.

Working Hypotheses

1, Inflow to Lake Naivasha comes from two streams - the Malewa and the Gilgil both from the north. Their perennial flows, due to their high altitude sources, make the lake fresh but its level responsive to global climatic patterns. Human impact upon the unpredictable nature of this discharge has become more severe over the past fifteen years, so that the level fluctuations now put at risk the resistance and resilience of the lake ecosystem.

2. The introduction of alien species to the lake - the first came in 1926 and the most recent in 1988 - has further damaged the resistance and resilience of the aquatic system.

3. Receding water levels and the impact of alien species upon the food web have combined to make possible the ecological and anthropomorphic destruction of the land-water ecotone.

4. Ecohydrological approaches to understanding these problems provide a firm basis for recommendations to management agencies.

Results

The pages following in this section illustrate the approaches to the problems and their solutions over the past twenty years.

(11)

UNESCO IHP-V 2.3/2.4 ECOHYDROLOGY - Core Study Lake Naivasha

LONG-TERM SIMULATION OF THE WATER LEVELS IN LAKE NAIVASHA

Authors

ROBERT BECHT

ITC, PO Box 6,750OAA Enschede, Netherlands SAMUEL MMBUIE

Ministry of Water Resources, PO Box 30521, Nairobi, Kenya Working Hypothesis

The simulation of the lake levels of Lake Naivasha is one component of the hydrological and environmental research carried out in the lake basin, under the working hypothesis 1. Expressed by Harper the main objectives are to:

1. Design a model, which allows the simulation of lake levels.

2. Establish a reliable water budget for the lake including the quantification of abstractions.

3. Investigate whether the lake level variations can be explained by purely meteorological/hydrological factors or by other (tectonic) factors.

4. Study the effect of the abstractions on the lake levels.

The Simulation Model

The lake receives water from rivers, direct rainfall, groundwater inflow and looses water by evapotranspiration and seepage. It is surrounded by highly permeable and porous aquifers, which dynamically interact with the lake.

A constant outflow to a deep aquifer system exists, keeping the lake fresh. The model was programmed in a spreadsheet. For monthly intervals the lake levels are calculated based on the stage-area-volume relationships, inflow, direct precipitation, groundwater outflow and interaction with the surrounding aquifer. The main calibration parameter is a constant groundwater outflow followed by the parameters controlling the lake-aquifer interactions.

Preliminary Results

The first graph shows that the lake levels can be accurately simulated for the full record period (193 1 -present). It clearly shows the effect of the abstractions which started in the mid-80’s and allows their quantification. The derived abstraction of 60 MCM tallies perfectly with abstraction

estimates based on

consumptive use and the area under irrigation.

The second graph shows the lake levels from 1900 to present, where the inflow series is generated from therainfall series. It clearly shows that the large variations of the lake levels before abstraction started can be explained by meteorological factors only.

MODEL BASED ON MALEW AND TURASHA FLOW

T 4 3

1892 1890 1888 1886 1884

Ott-54 Jun-68

I

Feb-82 Ott-95

I

- observed level . . . calculated level SIMULATED LAKE LEVELS BASED ON NAIVASHA RAINFALL (correlation)

,885 c _-~~~~--_. .--. -_-~ -~-- I

an 00 Sep-13 May-27 Jan-41 Ott-54 Jun-68 Feb-82 Ott-95 Jut-09

Ecohydrological Implications of this Study

The off take of water can now be quantified. The users of the lake - at catchment and at riparian spatial scale now have to agree a plan which conserves the essential ecological processes in the lake to allow the use to achieve sustainability.

(12)

UNESCO IHP- V 2.3/2.4 ECOHYDROLOG Y Core Study Lake Narvasha

THE PHOSPHORUS SOURCES TO LAKE NAIVASHA, KENYA

Authors

NZULA KITAKA

Department of Zoology, Egerton University, Box 536 Njoro, Kenya

DAVID HARPER Department of Biology, University of Leicester, UK KEN MAVUTI

Department of Zoology,

University of Nairobi, Box 30 197, Nairobi Working Hypotheses

I. That the ecotones of the lake and its inflowing rivers are degraded so that their ecological processes are no longer able to metabolise the nutrients.

2. That the major inflows to Naivasha - the Malewa and Gilgil- are thus sources of phosphorus to the lake.

3. That lakeside riparian land uses - horticulture, sewage effluent, urban runoff are major point sources.

4. That Lake Riparian land users may construct artificial wetlands that successfully mitigate nutrient input.

Methods

Analysis of inflow discharges and concentrations of phosphorus forms by standard chemical methods.

Results

During the start of heavy rains in November 1997 high total phosphorus quantities flowed into Lake Naivasha from its catchment rivers, originating in the middle reaches where human settlement is highest and agriculture is intense. During dry seasons the inflows are considerably reduced.

In the lake the impact of the inflowing swollen Malewa is apparent as a plume for up to one kilometre of higher P, suspended solids with lower conductivity. High concentrations are also found offshore of individual point sources.

The phosphorus concentrations passing through and leaving an artificially constructed riparian wetland taking runoff from one of the largest horticultural industries were reduced down to those of the lake water.

r

A: 1997

Distance from upstream (km)

The improvements available as a result of riparian ecotone construction need to be demonstrated on rivers and quantitatively estimated for catchment-scale effects. Additional riparian ecotones need to be constructed to receive the sewage effluent and urban runoff of the town of Naivasha, and other intensive horticultural enterprises.

(13)

UNESCO IHP- V 2.312.4 ECOHYDROLOG Y - Core Study Lake Naivasha

THE INTEGRITY OF THE NAIVASHA CATCHMENT STREAMS AND WETLANDS

Authors

MARK EVERARD

Director of Science, The Natural Step UK, 9 Imperial Square,Cheltenham, UK JACQUELINE VALE

Environment Agency, Bristol, UK ANTHONY KURIA

East African Natural History Society, Nairobi, Kenya

MICHAEL MAINA Department of Ornithology, National Museums of Kenya, Nairobi, Kenya

DAVID HARPER Department of Biology, University of Leicester, UK

Working Hypothesis

I. The riparian corridors of the catchment rivers provide functioning ecotones, which can be utilised, for the nutrient and sediment trapping to protect Lake Naivasha and maintain the processes of the lake basin.

2. Existing wetlands within the lake basin are intact and provide a biodiversity and functioning.

Preliminary Results

Semi-quantitative assessment using the River Habitat Survey technique demonstrates a diversity of habitats and erosion/sedimentation patterns. Flowering plant distribution along the river corridors appears most strongly correlated with altitude, whilst bird abundance and diversity correlate inversely with distance along river corridors away from the lake.

Wetland systems in the cat&rent have been defined and preliminary visits made, but several formerly significant headwater wetlands have been drained and converted to agriculture and the most significant change appears to be the almost total conversion of the Gilgil inflow wetlands since water levels declined in the early 1980s to irrigated agriculture swamp. The ecotone was formerly known as the ‘North Swamp’ and shown to be very extensive on 1950s maps (see location map on Harper’s page).

Sustainable management of the lake inevitably depends upon an understanding of the behaviour and pressures upon the catchment, since changes in land use patterns and intensity even remote from the lake itself can significantly influence hydrological, chemical, microbial and biotic inputs to the lake. They can also adversely affect habitat diversity and biological diversity, as well as the long-term economic viability of development in the catchment. A full inventory and quantification of the values of ecotones and wetlands in the system is urgently required and underway.

(14)

UNESCO IHP- V 2.312.4 ECOHYDROLOG Y - Core Study Lake Nawasha

A “SEQUENTIAL WETLAND”

AS A PRIMARY TOOL FOR THE RESTORATION OF NAIVASHA

Authors

MACIEJ ZALEWSKI

International Centre for Ecology, Polish Academy of Sciences, Warsaw DAVID HARPER

Ecology Unit, Department of Biology, University of Leicester, UK

Study Area

Lake Naivasha (Kenya), an important water source for domestic use for over 100 000 people and intensive horticulture (15 % of Kenya’s horticultural export).

Working Hypothesis

The sharp increase in human population during last 20 years, from 20 000 to 200 000 people in the river basin, has resulted in degradation of the catchment cover. Thus to a great extent, increased erosion and nutrient transfer by the river system into the lake which has affected eutrophication, illustrated by the reduction of water transparency.

To reduce and reverse the eutrophication processes, a primary measure could be the reduction of nutrients, organic and mineral matter provided by the Malewa River. As the main part of the annual nutrient load is transported by fluxes during heavy rains, this fraction of the flood should be directed and retained in a constructed Treatment Wetland, where sedimentation processes and biogeochemical trapping should significantly reduce the load entering the lake. The evidence from the scientific literature indicate up to 80%

reduction of suspended matter and over 50% of total phosphorus could occur during more than three days of residence time.

Methods

This constructed wetland should possess the sequential structure to reduce economic costs of eliminating land from agricultural use. A first wetland, which will have the most complex structure and highest efficiency of organic matter and nutrients trapping will be near to the river, and should have its capacity adjusted to be sufficient to cope with the volume of water from the frequent, short, intensive rains.

The capacity of a second stage should be adjusted to cope with heavy rains during the annual rainy season.

The land, which is periodically flooded, should be used for less demanding agricultural production. The capacity of a third stage could be calculated to contain the amount of rain in the intensive rainy periods that occur every 3 years. The land at a third stage might be used for various types of agricultural activities but with some limitations. Regular vegetation cropping will be necessary to maintain the high trapping capacity of first stage.

The second and third stage could be agriculturally productive without fertilisation, as the nutrient and organic load collected in the whole Malewa catchment should be sufficient to maintain high agricultural activities.

For elaboration of the quantitative model of constructed wetlands it is necessary to:

1. Evaluate hydrological variability of flow at the river.

2. Evaluate quantity of nutrients, organic matter and minerals at different flood intensities and at various stages of floods.

Such a “sequential wetland” could be the first step in an integrated strategy of restoration of Lake Naivasha. The conversion of the Malewa delta in its riparian area into the sequential wetland would be the most feasible and

(15)

UNESCO IHP- V 2.3/2.4 ECOHYDROLOG Y - Core Study Lake Naivasha

METAL CONTAMINATION

OF SEDIMENT IN LAKE NAIVASHA AND ITS CATCHMENT RIVERS

Authors

HAKAN TARRAS-WAHLBERG Swedish Geological AB, Stockholm, Sweden

DAVID HARPER Department of Biology, University of Leicester, UK Working Hypothesis

The Lake Naivasha area has experienced rapid development in latter decades, most importantly the rise of high intensity agriculture. The agricultural developments have, in turn, caused population increase as well as having spawned some auxiliary industrial activity.

The hypothesis was that these developments have not led to metal contamination of the sediment in the Lake itself or its catchment rivers. A programme of sediment sampling and geochemical analysis was performed to confirm this.

Methods

The investigations were performed in July and August 1997, and comprised two parts:

1. Sampling and analysis of catchment rivers’ sediment:

Ten samples of sediment were collected, sieved through a 2 mm sieve, air-dried, and analysed by X -Ray Fluorescence spectrometry (XRF).

2. Sampling and analysis of lake sediment:

Two sediment cores were taken, one from the central lake and one from near a papyrus swamp in the northern part of the lake. Ten sub-samples were then collected from each of the identified stratigraphic units of the two cores. The geochemistry of the sub-samples was determined by Plasma emission spectroscopy (ICP-AES), following digestion in Aqua Regia.

Results

Generally, the metal contents of the sediment are low, and there is no indication of significant metal contamination. Only two samples of Lake sediment contained somewhat elevated metal contents, most importantly cadmium concentrations of 7 mgikg and 6 mgikg respectively. Further work is needed to determine whether the sources of cadmium are natural or anthropogenic.

AVERAGE CONCENTRATION OF SOME IMPORTANT METALS IN RIVER AND LAKE SEDIMENT IN THE LAKE NAIVASHA AREA

Concentration

(mgM) cu Cd Ni Pb Zn

River sediment 10 11 14 153

Lake sediment 16 <3 27 ~6 146

The concentrations of metals in sediments in Lake Naivasha and its catchment rivers are low, and are probably representative of natural background conditions. However, some further work is motivated to elucidate the sources, pathways and bioavailabilty of somewhat elevated concentrations of cadmium in Lake sediment.

(16)

UNESCO IHP- 1’ 2.3/2 4 ECOHYDROLOG Y - Core Study Lake Naivasha

INVESTIGATIONS OF THE SEDIMENT STRATIGRAPI-IY OF LAKE NAIVASHA

Authors

HAKAN-TARRAS-WAHLBERG

Swedish Geological AB, Stockholm, Sweden DAVID HARPER

Department of Biology, University of Leicester, UK

Background

Lake Naivasha’s waters are increasingly used for irrigation, and considerable areas of the Lake catchment have been converted for agricultural use. Additionally, lake levels fluctuate for natural reasons, and local tradition says that there was a time in the 19th century when the lake was completely dry.

These natural fluctuations, coupled with increasing human influence, have made the lake and the ecosystems that are dependent upon it, unstable and fragile. Investigations of lake sediment stratigraphy provide evidence of the effects of past water level and land use changes and, thus, may provide important insights needed for the present wise management of the lake.

Methods

Eighteen piston core, sediment sections were sampled at evenly spaced locations of the Lake. The sediment cores were extruded and each stratigraphic unit’was described in detail with regards to its physical and biological characteristics.

Results

An idealised one-meter stratigraphic section of the central Lake Naivasha basin was established. The section is interpreted as stretching back in time to the 16’h or 17’h century AD, and contains layers laid down during high water levels, layers laid down when the lake was a shallow swamp as well as a distinct non-conformity caused by the lake drying out completely. Investigations of sediment characteristics at the sediment-water interface show that present-day sedimentation dynamics are governed by the presence of point sources of sediment, and the effects of wave-induced re-suspension of sediments. Sediments introduced by rivers in the north are transported in easterly and southerly directions, and are eventually deposited in the central and southern parts of the lake. The sedimentation rate in the central lake was found to be about 1 cm per year, whereas in the south, the rate may be as high as 3 cm per year. Some sedimentary deposition is also occurring in areas in the north which are protected by Papyrus vegetation. In the north-eastern part of the lake, historic low lake levels have led to the formation of a wide spread mineralised clay layer, present at or near the water-sediment interface.

The stratigraphic evidence confirms that lake levels have fluctuated widely in the past, and thus, that the instability of the lake is in part natural. The changes in lake levels have been accompanied by distinct changes in the vegetation of the lake as reflected by varying amounts of swamp vegetation in different stratigraphic units.

The lake’s papyrus swamps are shown to be areas of sedimentation and, thus, their continued existence is vital in order to counteract tendencies for increased turbidity of the lake’s water. The possible existence of a continuous clay layer in the north-eastern part of the lake may make impossible a previously postulated ground water recharge in this part of the lake, and therefore, other locations or other mechanisms for groundwater recharge may have to be sought.

(17)

UNESCO IHP- V 2 30 4 ECOHYDROLOG Y Core Study Lake Naivasha

THE ENVIRONMENTAL HISTORY OF LAKE OLOIDIEN, KENYA:

IMPLICATIONS FOR THE MANAGEMENT OF RIFT VALLEY WATER RESOURCES

Authors

DIRK VERSCHUREN CHRISTINE COCQUYT

University of Gent, B-9000 Gent, Belgium JOHN TIBBY

Monash University, Clayton, Victoria 3 168, Australia PETER R. LEAVITT

University of Regina, Regina, SK S4S OA2, Canada C. NEIL ROBERTS

University of Plymouth, Devon PL4 8AA, UK Study Area

Rift Valley lakes

Oloidien and Naivasha, Kenya.

Working Hypothesis

The past history of the climate, through Rift Valley Lakes’ sediment history, provides a guide to present-day management, through an explanation of the range of wetland environments which lake level change produced.

Results

Paleolimnological analysis of sediment cores from Lake Oloidien produced a detailed chronology of climate- driven environmental change since the early 1800s. During much of the 19th century and between about I940 and today, Lake Oloidien was shallow, separated from Lake Naivasha, and inhabited by the productive algal and invertebrate communities typical of moderately saline lakes. Confluence with Lake Naivasha between 1890 and the late 1930s flushed dissolved salts out of Lake Oloidien and created a temporary freshwater phase during which the lake shore was fringed with papyrus swamp and extensive beds of submerged macrophytes, and the open water algal flora was dominated by diatoms and green algae. The timing and of these ecological changes in Lake Oloidien implies that agricultural development of the Rift Valley by British colonial settlers in the first two decades of the 20”’ century coincided with an unusual plentiness of water-resources, a remnant of high rainfall during the 1880s and early 1890s out of balance with the long-term average rainfall and evaporation. Episodes of drought during the 1920s 1940s and 1950s may have been experienced as exceptionally severe only because the evaluation of land and water resources on which colonial agricultural development had been based was biased by the earlier imbalance between water-resource availability and climate. Lake-level decline and rising salinity of Lake Oloidien during the early 199Os, as well as the opposite trends that followed El NiRo-related rainfall in 1997, are mostly a direct consequence of the lake’s hydrological response to natural rainfall variability on interannual and longer timescales.

Heavy anthropogenic water use around Lake Naivasha and the diversion of Malewa River water for cropland irrigation and industry in the Lake Nakuru area increasingly compromises the ability of high-rainfall years to compensate for water losses incurred during periods of relative drought. The ongoing transition of Lake Oloidien to a soda-lake environment is thus symptomatic for the threat that human activities may pose to the survival of Lake Naivasha and other shallow lakes and wetlands in the Kenya Rift Valley. An integrated strategy for long- term management of these aquatic ecosystems is needed to protect their diversity of natural and semi-natural habitat while maintaining their important economic functions. Sustainable development of dryland regions in tropical Africa requires general recognition that water is not a fixed resource but varies strongly at timescales from seasons to centuries.

(18)

UNESCO IHP- V 2.312.4 ECOHYDROLOG Y - Core Study Lake Nawasha

WETLAND SOILS OF LAKE NAIVASHA

Authors W. SIDERIUS ITC Soil Division,

Enschede, The Netherlands G. J. URASSA

National Soil Service, Tanga, Tanzania

SCHEMATIC CROSS-SECTION OF THE GROUNDWATER LEVEL AS CAUSED BY THE RECENT LAKE TRANSGRESSION

dry ^10m water ltne flooded

- no. oloh*u”am,7

1 2 3 8 5 6

10cm

I’ , ,,’ / ,,’ ,/ ./’ ,/,,,’ ,_,’ ‘,/’ ,,,

S01l moisture - - * _I’. ,,,’ ,, / ,/

_, _A , _, ,’ _, , _, ,’ _A ,, _, ’ lncreas~ng wth depth ,-’ ,,’ /’ , ,’ , .,’ , ,’ , I.’ ,_,’ .’ ,_ ,

c-

-+- -- __-- -- ,__I __ ’ .’ _,. N’,,, / ,,,

, ,I _.

__--, I , ,,’ ,*’ ,_’ ,_,. ,__I _,* , _‘_,

sediments Consolidated tuff layer

J

Methods

In 1998, just after the El fiino-Southern Oscillation had caused a lake level rise, wetland soils which had been recently flooded were analysed for Eh, pH and Ece. 164 samples were taken from in 6 transects in the eastern and southern shores, where horticulture and urban development are greatest.

Results

Soils are deep and moderately well to poorly drained. Texture is sandy clay, giving way 20-50 cm to sandy clay loam. They are

REDOX POTENTIAL-SOIL DEPTH FUNCTIONS FROM THE DRIER AREAS (observations 1,2 and 3)

TO THE FLOODED ZONE (observations 5 and 6).

Observation 4 taken at the waterline (lake edge)

developed from lacustrine sediments formed from volcanic materials. Cation exchange capacity varies between 19-43 me/l00 g soil; pH between 7.0-8.0.

The soils are non-saline (Ece < 4.0 mS), with topsoils having the lowest salinity due to leaching.

Redox potential decreases rapidly in flooded soils, marking anaerobic conditions, which indicates the process of reduction to become active in a short period. The cluster between pH and’Redox potential is similar to mature soils, which may induce a greater mobility of cations such as iron as soil acidification develops.

Redox potential (mV)

Flooded Dw

-100 -50 0 +50 +100 +150 +200

I I I I I I

i ---- _ _ I

MOBILITY OF Fe IN THE WETLANDS SOILS AT LAKE NAIVASHA

>^ 800 r

5 600

Y 5 400

5 200

D P 8 0 PI -200

6 0 10 12

PH

Cateaow:

Normal soils Waterloged soils Wet soils

Studies on the ecotones of this lake have largely focussed upon the biotic components, with little attention paid to wetland soils. Their position warrants further study however, as they act as the important buffer for surface water runoff because of the porous nature of the soil which means that little water reaches the lake by overland flow.

(19)

UNESCO IHP-V2.312.4 ECOHYDROLOGY - Core Study Lake Naivasha

ENVIRONMENTAL MAGNETISM AS A TRACER FOR SOIL EROSION ALONG THE SOUTHERN SHORELINE OF LAKE NAIVASHA

Authors ROS BOAR

School of Environmental Sciences, University of East Anglia,

Norwich UK DAVID HARPER

Lake Naivas ha

high erosion risk no papyrus

Working Hypothesis

1. Intact ecotone papyrus swamps intercept soil particles that have eroded from riparian land.

2. Papyrus swamp is impermeable to large (>2000 pm), medium (2000-63 pm) and small (~63 pm) soil particles.

3. The magnetic properties of sediment particles in Lake Naivasha will reveal their source.

Methods

1. Wet sieving and dry weighing of size fractions of top soil and lake sediment sampled at intervals along three transects running from upland, through swamp and into open lake.

2. Mass specific magnetic susceptibility and magnetic resonance measurements of the < 63 pm size fraction of topsoil and lake.

Results

1. At high risk of erosion, large (>2000 pm) particles are retained by marginal papyrus swamp.

2. At high risk of erosion in the absence absence of marginal papyrus, large particles (>2000 pm), sands (2000 pm - 63 pm), silts and clays (<63 pm) enter the lake.

3. The magnetic susceptibility of the ~63 pm fraction varies from upland to lake sediment.

4. Magnetic properties of sediment along the southern shore of the Lake where papyrus has been removed suggest that silts and clays in lake sediment have come from erosion of the hinterland.

3

.5 t

+ t l A

high erosion risk no papyrus

‘I

1”1 I”

@ = 0.8545

* .5 N = 20 PC

0.001

other sites where papyrus is intact

n ^f83,

u,

0 100 200 300 400 500

Distance along transect from hinterland (m)

-I

600 700

1

Conservation or the recreation of a continuous papyrus ecotone, to intercept fine particles, is of the highest priority. Otherwise surface runoff may carry into the lake ecosystem contaminants from horticultural land and

(20)

UNESCO IHP- V 2.312.4 ECOHYDROLOG Y - Core Shdy Lake Naivasha

THE SEVERE LOSS OF THE AQUATIC PLANT COMPONENT OF THE LAND-WATER ECOTONE AT NAIVASHA; ITS CAUSES AND POSSIBLE RESTORATION

Authors

DAVID HARPER

ANNE-CHRISTINE GOUDER

Faculties Universitaires de Gembloux, Belgium CHRIS ADAMS

Environment Agency, Ipswich, UK PHIL HICKLEY

Environment Agency, Worcester Road, Kidderminster, UK

I water lily beck 1 Submerged plant beds ---

= Emergent Cvperw PaPVPUS WllmP

0 4 ml

Working Hypothesis

For the past twenty years there has been dramatic fluctuations in the submerged, floating-leaved and free- floating plants in the lake. Since 1987 we have worked on the hypothesis that these fluctuations are caused by the grazing impact of the alien Lousiana crayfish, Procambarus clarkii, introduced as a potential fisheries crop in the early 1970s.

Methods

Submerged plant abundance is recorded by spot sampling with a grapnel followed by identification and GPS location. Crayfish abundance is recorded in floating plant quadrates and also as prominence value in the gut contents of large-mouthed bass from experimental gill netting.

Results

Both the emergent component of the ecotone, dominated by Cyperus papyrus, and the submerged/floating component have been severely reduced. The emergent component has primarily been lost through agricultural destruction as a result of the legal permission to cultivated Riparian land (see Enniskillen) during low lake levels dating back to 1929. The floating-leaved and submerged component has been destroyed by grazing impacts of crayfish, whose abundance is best measured by their value as food to large-mouthed bass (see Figure).

LAKE NAIVASHA

AQUATIC PLANTS AND CRYFISH ABUNDANCE 1987-1998

500 1000 lwo 2000 2500 3000

Aquatic plant cover (ha)

1

High water levels, such as those experienced between December 1997 and February 1998 as a consequence of the effect of heavy rains following the El Nino, mitigate the problem of emergent C. pamrus for as much as 10 years. The current ecotone structure of C. papyrus all germinated during May 1988 when there were heavy rains causing a I vertical m water level rise. However the submerged macrophyte destruction can only be mitigated by some kind of biological control over the crayfish. Such biomanipulation includes removal by a healthy bass population (see Hickley), a diverse piscivorous bird population (see Childress), a strong crayfish industry (see Smart) or by the periodic cycling which occurs with crayfish populations. If the latter, then the processes of the ecotone are unlikely to recover to provide effective protection from catchment sediments and nutrients.

(21)

ENVIRONMENTAL CONDITIONS IN THE LAGOONS AND SWAMPS OF THE NAIVASHA ECOTONE

Author

DAVID HARPER

Working Hypothesis

The combination of lake level changes of several metres over the past twenty years, with destruction of papyrus by humans, destruction of submerged plants by crayfish and the influx of floating alien plant species, have created rapid and unpredictable changes in the lake ecotone. Recognising the ecotone’s development causes and understanding the physical conditions created is a pre-requisite to understanding how efficiently it may function in the lake ecosystem.

Methods

Physicals recording of size, depth, position of lagoons using a GPS were made, in relation to lake level.

Transects of vegetation from land to water were measured. Temperature and oxygen were recorded in water of various depths throughout the day. Subsequently satellite images of key periods were obtained and studied using GPS.

Preliminary Results

Lagoons are created in a number of different ways during high or rising water levels: a) floating C. papyrus islands becoming stranded in shallow water (the dominant method up to the end of the 197Os), b) high water level flooding through rooted C. papyrus and open lagoons forming on the landward side of the C. papyrus stand (dominant in the early 198Os, the early 1990s and 1998), c) inside floating mats of Eichhornia or Salvinia where some obstruction retains them (the mid-1980s).

These lagoons are important parts of the changing ecotone. They provide spawning areas for fish, feeding areas for juveniles, germination areas for submerged and floating-leaved plants in newly flooded land (before they are reached by crayfish) as well as feeding foci for aquatic birds.

Temperatures rise to 30°C in the early afternoon and de-oxygenation may occur.

SATELITE IMAGE FROM 1975 SHOWING LAGOONS IN THE NORTH, WEST AND

SOUTH OF THE LAKE

The processes inherent in a natural ecotone is/will be severely disrupted by these changes in structure. Future measurement of processes, such as nutrient relationships, will be able to assess the ecosystem-scale importance once the spatial and temporal extent of these structures is fully quantified.

(22)

UNESCO IHP- V2.312.4 ECOHYDROLOG Y - Core Study Lake Narvasha

THE DYNAMICS OF FLOATING PLANT MATS IN LAKE NAIVASHA

Authors CHRIS ADAMS

Environment Agency, Ipswich, UK DAVID HARPER

CLASSIC SEQUENCE OF DRAWN-DOWN SUCCESSION FOR LAKE NAIVASHA

trees/shrubs

herbs sedges floating

/g&es plants submerged plaflk After Gaudel (1977). AquatIc Botany 3. 12.47

Working Hypothesis

Hydroseral succession was well described by Gaudet in the 1970s when the ecotone of the Lake Naivasha riparian zone was almost natural. Since that time it has been invaded by alien floating species as well as undergoing degradation as a consequence of human activities as low discharges. Our hypothesis is that floating mats of Salvinia and Eichhornia have no effect on the dynamics of hydroseral succession in Lake Naivasha.

Methods

I. Percentage cover in 4 m x 2 m quadrates was recorded in up to 155 sites around the edge of the Lake at intervals over the period 1988 to 1998.

2. All plants were identified to species. Species have been grouped into: herbaceous plants, grasses and sedges, floating species and shrubs.

Results

Lake Naivasha currently supports four main species of submerged macrophytes (Potomogeton pectinatus, P.

schweinfurthii, P. octandrus and Naias horrida), together with the floatin, 0 water lily Nymphaea caerulea when its growth is possible (see previous project). Since the mid 1970s there have additionally been floating mats of Salvinia molesta and, since 1998, of Eichhornia crassipes (water hyacinth).

Salvinia molesta dominated these mats until 1989 when 81% of 155 sites surveyed had a percentage cover of >

75% Salvinia. By 1993, when 132 sites were surveyed, only 5% of sites reached the same high density of floating Salvinia. However, Eichhornia crassipes had by then reached > 75% cover in 63% of the sites.

There has been a development of plant types between 1988 and 1998. Moreover, an increase in the species richness of herbaceous plants and of grasses and sedges is apparent.

L J

DEVELOPMENT OF PLANT TYPES IN FLOATING 251 ElCHHORNlA MATS (1988 - 1998) 20

15 10 5 0

Sep-88 Jan-89 Apr-89 Apr-93 Aug-97 Aug-98

shrubs herbs

grasses and sedges floating species

The classic, ordered sequence, of plant succession from open water to shoreline shrubs still occurs in paces with little change in species lists. However, the movement of floating mats re-distributes seral stages so that plant succession no longer follows a classic spatial sequence. Introduction of floating plants has not necessarily been undesirable, since there is no evidence of any decline in species richness of mid-successional plants, particularly of herbs, grasses and sedges. The starting hypothesis is thus supported in that temporal sequences

(23)

UNESCO IHP- V 2.3i2.4 ECOHYDROLOG Y - Core Srtrdy Lake Narvasha

INVESTIGATIONS OF FLOATING VEGETATION, EICHHORNIA CRASSIPES AND SAL VINIA MOLESTA, ON L&SE

NAIVASHA

Authors

NILS TARRAS-WAHLBERG tiKAN TARRAS-WAHLBERG

Kyrkeryds Lillegird, 35.5 92 Vaxjij, Sweden DAVID HARPER

Study Area

The floating water plants Salvinia molesta and Eichhornia crassipes were introduced to Lake Naivasha, Salvinia in the early 1960’s and Eichhornia in the late 1980’s. Both these alien plants have periodically thrived and, consequently, attempts have been made to eradicate them. Control of Salvinia was unsuccessful until the early 90s when, roughly after the introduction of the weevil Cyrtobagus salviniae, the plant almost totally disappeared.

It is, however, unclear whether the disappearance of Safvinia was caused by the weevil or by other concomitant environmental changes. Eichhornia is now the Lake’s most important floating plant, and it is feared that the propagation of Eichhornia may convert the lake into a marshland. Therefore, a weevil (Neochetina) has been introduced to the lake in an attempt to control Eichhornia, although whether this weevil has been successfully established is not known.

Working Hypotheses, Methods

A survey of the floating plant vegetation of Lake Naivasha was performed in September 1996. The aims of the survey was twofold:

1. To study the distribution of floating vegetation on the Lake and in so doing, suggest mechanisms that may explain identified changes in plant distribution.

2. To investigate the mesofauna associated with the Salvinia and the Eichhornia vegetation.

Results

Previous work has shown that Salvinia exposed to the windy and wavy conditions of an open lake quickly perishes. During this survey, Salvinia was only found in a sheltered area at the northern end of the lake.

Furthermore, no individuals of the weevil Cyrtobagus salviniae were found in these Salvinia plants. These findings strongly imply that the weevil has not caused the disappearance of Salvinia from the lake, and that instead environmental change, in turn related to lake level change and human activity, have led to a loss of suitable Salvinia habitats in the main Lake. No individuals of the weevil Neochetina were found in Eichhornia plants, implying that the weevil’s introduction has been unsuccessful. However, Eichhornia plants do support a rich mesofauna, concentrated in the layer in between the plants’ leaves and aquatic roots. In this zone, dead plant material is continuously being broken and soil is formed. The most important organism in this process appears to be the earthworm Almia emini.

The introduction of weevils seems to have had negligible effects on the Lake’s floating vegetation. Instead, environmental changes (water level change and human activity) appear to have caused the disappearance of Salvinia from the lake. Eichhornia plants represent nuclei for soil-forming processes. Such processes may accelerate the loss of open water areas, and consequently represent a different threat to the lake. Therefore, further investigation of the Lake’s Eichhornia-based ectonal ecosystem, aimed specifically at the rate, extent, stability and functioning of soil formation on floating mats, are needed.

(24)

UNESCO IHP-1’2.3/2.4 ECOHYDROLOGY Core Study Lake Narvasha

I I

THE SPATIAL AND TEMPORAL

^36'20 ^36'25

DISTRIBUTION OF LOUISIANA

CRAYFISH

Authors

ANNE-CHRISTINE GOUDER DE BEAUREGARD SOPHIE SCHMIDT

Lake Naivasha

Faculties Universitaires de Gembloux, Belgium ANDY SMART

STEPHANIE COLEY

Cornwall College, Cambourne, Cornwall, UK DAVID HARPER

Department of Biology, University of Leicester, UK KAMAU MBOGO

36'20 36'25

I I

Kenya Marine and Fisheries Institute, Naivasha Working Hypotheses

For 25 years, aquatic vegetation has been changing in time and in location on Lake Naivasha. The modifications observed on the plant communities appear at the same time the Louisiana crayfish population began to be important and widespread all over the lake. The hypotheses are as follows:

1. The crayfish population is responsible for the changes observed in the macrophytes.

2. Crayfish have a spatial and a temporal impact on the aquatic vegetation dynamics.

3. Crayfish eat and damage plant communities, more especially submerged macrophytes.

Methods

Long term monitoring :

1. Annual crayfish survey (hand-net, quadrat, trap) all over the lake shoreline.

2. Annual mapping of crayfish population (GPS).

3. Annual mapping of vegetation communities (GPS).

Experimental design, Preliminary Results

Crayfish are widespread on the lake. They are present on the shoreline particularly where macrophytes are established. A high crayfish density are found in floating/grounded plants of Cyperaceae or Eichhornia crassipes. Very few crayfish are found along a cleared shoreline or next to small clumps of vegetation.

1992

1

Eichhornia

Salvinia Inside the vegetation, crayfish live in

different habitats according to their size.

Juveniles are found in Salvinia molesta Mitch. mainly, then in Eichhornia crassipes when they are bigger. Most of the adults are found in the papyrus fringe.

o-9 9 11 -19.9 20.0-29.9 30.0-39 9 40.0-49 9 50 O-59.9 60 O-69 9

Size class

The relation between the papyrus fringe and the submerged plants was linked with crayfish density in 1998.

Submerged macrophytes were absent when the shoreline was composed of Papyrus spp., thus where the crayfish density is higher. And submerged macrophytes were present when there was no papyrus fringe, in other words where the crayfish density was low. The dynamics of Papyrus spp., Eichhornia crassipes and Salviniu molesta seemed to be independent of the crayfish population. Laboratory experiments and ecological survey on the lake tend to show that there were little or no relations between them.

In order to fully understand the crayfish dynamics in the aquatic ecosystem we need to understand more clearly its food base, its effect upon plant species and its predators. This gives us the best opportunities for restoration of

(25)

UNESCO IHP- V 2.312.4 ECOHYDROLOG Y - Core Study Lake Naivasha

THE POPULATION STRUCTURE AND DISTRIBUTION

OF LOUISIANA CRAYFISH

(PROCAMBARUS CLARKII, GIRARD)

AT LAKE NAIVASHA, KENYA

Authors

STEPHANIE COLEY ANDY SMART

Cornwall College, Cambourne, Cornwall, UK DAVID HARPER

KAMAU MBOGO

Department of Biology, University of Leicester, UK Introduction

The changes in the ecology of Lake Naivasha have been monitored extensively over the last decade and changes in vegetation have been linked with changes in the population of introduced crayfish Procambarus clarkii. The distribution of the vegetation around the lake is possibly linked to the crayfish.

Working Hypothesis

There are no differences between the temporal and spatial distribution of crayfish of different size and sex around Lake Naivasha. Measurement of carapace length is a valid measure of size and weight of crayfish at Naivasha.

Methods

1. Animals were collected from around the lake, sexed, weighted and measured. Analysis of measurements showed relationships between field measurements and actual dimensions of animals.

2. Different locations around the lake were sampled and the sex ratios and size ratios examined.

3. Mark-recapture was attempted using fish traps set along the edge of the shore.

Preliminary Results

1. Male and female crayfish show a different relationship between carapace length and size and weight. A relationship between carapace length and tail weight was established.

2. The overall sex ratio was established as 1: 1 and did not vary from year to year; berried females and small juveniles were not found throughout the year suggesting that breeding is no longer continuous as previously reported.

3. Mark-recapture of crayfish over two periods in 1992 and 1994 proved inconclusive, despite large numbers of animals marked few recaptures were recorded suggesting that animals may have no fixed areas in which they live.

Ecohydrological Implications of this Study Variations in the temporal and spatial distribution will affect the bass fishery and may have implications for the aquatic macrophytes. An understanding of the crayfish ecology is central to the understanding of the lake’s food web.

J” I I

4Oi

I

ii ‘il, *i&c (I

Carapace length

60 , -1

6

Total length

5 t &

+g 4-.

.” -I

4 ’

t 3

w

+*+I t +

- 2

I-”

⁺⁺

l * .

1 It@., ’

/

m

‘3 tb

2d

3b

4b d

(26)

THE DISTRIBUTION OF LITTORAL ZONE MACROINVERTEBRATES

AT LAKE NAIVASHA, KENYA, BETWEEN 1992 AND 1994

Authors A.C. SMART S.J. COLEY D.M. HARPER

Department of Biology, University of Leicester, Leicester LEl 7RH, UK

Introduction

The changes in the ecology of Lake Naivasha have been monitored over the last decade. Benthic and littoral macro-invertebrates were surveyed in detail during 1982-1984 and subsequent changes in vegetation and lake levels have been recorded in detail. A repeat survey of the lake using similar methods was undertaken during l992- 1994 to establish whether any changes had occurred during this period.

Working Hypothesis

There is no difference in the spatial distribution of littoral macro-invertebrates around the lake shore and the density of macro-invertebrates on different macrophytes does not vary with species.

Methods

I. Semi-quantitative samples of macro-invertebrates were removed from a series of stations around the lake shoreline.

2. Semi-quantitative samples of macro-invertebrates were removed from the three main aquatic macrophytes and the two main species of floating vegetation.

Preliminary Results

1. Taxa varied around the lake with vegetation and substratum. Notable differences were the presence of a

Chonchostracan in both 1992 and 400 1994 and the absence of the majority

of the Hemipteran fauna and

Anisoptera on the main lake. 300 2. The macro-invertebrate community ;

varied considerably on different $ 2oo plants; Najas and Potamogeton z pectinatus dominated by Micronecta

and chironomid larvae; Potamogeton 100 schweinfurthei dominated by

Ostracoda, snails and flatworms.

n

MACRO-INVERTEBRATES

per 1009 air dried macrophytes

0 Potamogeton

schweinfurthii

The changes in macro-invertebrate fauna will have implications for the fish and fishery on the lake.